1. Field of the Invention
The present invention relates to a multilayer intermediate product impregnated with polyurethane, useful for the preparation of high quality artificial leather that has a suede-like appearance and that is soft, highly formable, and characterized by an enhanced mechanical resistance. The present invention also refers to a process for obtaining the above-mentioned multilayer intermediate product and the final suede-like leather deriving from said intermediate product. Typical uses of said artificial leather are the coverings of padded furniture, car interiors, and car seats.
2. Description of Related Art
Synthetic artificial leathers having a suede-like appearance are known in the art and are characterized by a surface having a high density of microfiber and a matrix capable of binding the microfiber structure.
The microfibers forming these types of materials are typically polyester- and/or polyamide-based, and the binding matrix is typically a polyurethane.
The binder not only holds the microfibers forming the non-woven fabric, but may also increase the mechanical properties and tear resistance of the composite structure. At the same time, the binder has a negative effect on softness and formability. Accordingly, the amount of binder typically used in high quality artificial leather is included within a range that prevents such a negative effect from being excessively pronounced.
Processes for manufacturing high quality artificial leather with a suede-like appearance are disclosed in patents EP0584511, U.S. Pat. No. 3,531,368 and U.S. Pat. No. 3,716,614, and can be summarized as follows:
1. Spinning of a bi-component fiber of the “sea-island” type, in which the “island” consists of polyester and/or polyamide and the “sea” consists of a polymer that does not mix with the island component and that can be dissolved in suitable solvents of the organic or inorganic type. The microfibers obtained after the dissolution of the sea component typically have counts lower than 0.5 den.
2. Preparing a felt characterized by predefined density and unit weight values, through a mechanical needling process that interfaces the microfibers obtained in step 1 with each other.
3. Impregnating the felt with a binder capable of holding the “islands” during the subsequent elimination phase of the “sea” component. This binder, which also has the function of adequately reinforcing the felt, allowing immersion in the solvent used for eliminating the “sea”, can be of two different types. The first type is typically based on polyvinyl alcohol, to be removed in a subsequent phase of the process, and the second type is typically based on a polyurethane which, even after the subsequent phases of the process, remains partially or totally in the final compound. The intermediate product obtained from this step is identified as “SR”.
4. Dissolving the “sea” component in a suitable organic solvent (normally trichloro ethylene) or an inorganic solvent (an acid, a basic aqueous solution, or simply hot water) to generate a microfibrous material. The intermediate product obtained from this step is identified as “D”.
5. Impregnating the above-mentioned microfibrous material with a solution of polyurethane (PU) in organic solvents (dimethyl formamide, or DMF). Alternatively, such an impregnation can be performed with polyurethane in emulsion or aqueous dispersion.
6. Eliminating of the binder used in step 3 (if that binder is not PU). The intermediate product obtained from this step is identifies as “IE”.
7. Dividing the bi-component laminate (“island” component+PU) into two equal parts by means of a longitudinal cut parallel to the surfaces.
8. Surface grinding by means of suitable treatment with abrasive paper, to provide the structure with a suede-like appearance.
9. Final dyeing of the product.
One of the expedients often used for widening the application range of artificial suede-like leather is combining the same with various kinds of supports capable of making the material compatible with applications requiring a higher mechanical resistance and/or different characteristics of formability, body, etc.
The simplest and most widely-used method for combining the microfibrous substrates with the different supports is to couple them with textile supports, by using suitable adhesives, after the dyeing process. Dyeing typically precedes coupling with various supports because it is difficult to identify adhesives, which can resist the severe process conditions used for dyeing artificial leather of the suede-like type and which, at the same time, allow a sufficient softness of the product to be preserved.
Because of this limitation, it is the impossible to obtain a final product having both sides dyed in the same dyeing process, thus characterized by a more or less pronounced similarity in the appearance of the two surfaces.
These differences are also due to the common use, as supports, of materials having a significantly different structure from that of the suede-like surface. Usually, in order to give the product the physico-mechanical characteristics necessary for the different applications, the supports are various types of textile structures having a visual appearance which is extremely different from that of the microfibrous surface with a non-woven structure.
Another method used for effectively combining the microfibrous substrates with the different supports used and for overcoming the above-mentioned drawbacks is to “bind” the various layers in an operating phase, which is “upstream” of the dyeing process, without making use of the above-mentioned adhesives. A technology of this type is disclosed in U.S. Pat. Nos. 4,368,227 and 5,112,421. Both patents teach the use of “multilayer” structures consisting of a combination of a microfibrous non-woven fabric with a traditional fabric or knitted fabric.
In these patents, the linkage between the various layers of the composite product is provided by interlacing the fibers, forming the same layers, by means of suitable needling processes.
The limitations in the solution proposed by U.S. Pat. No. 4,368,227 are mainly caused by the required use of large quantities of the polyurethane binder, due to the use of very short fibers (≦10 mm) interlaced by means of water needling. Water needling does not necessarily provide a degree of interlacing that enables the use of small amounts of polyurethane binder in the subsequent impregnation phase.
With the proposed solution, on the other hand, mechanical needling is not possible, which would otherwise ensure a higher interlacing between the microfibers and between the layers, since mechanical needling would cause an excessive structural weakening caused by the breakage of the fibers forming the various layers of the composite material.
A limitation in the teachings of U.S. Pat. No 5,112,421, which uses longer fibers (>20 mm) and therefore requires smaller amounts of binder for retaining the fibers, is the requirement for highly twisted yarns for producing fabrics representing the reinforced layer. This is caused by the required overlapping of the single layers, whether microfibrous and not, and by the subsequent needling operations, which lead to the production of an intermediate product characterized by suitable values of thickness and density.
The use of highly twisted yarns has the purpose of preserving these compounds from an excessive structural weakening but, on the other hand, has an effect on the visual appearance of the finished product. Needling density is required to be high and conveys a significant number of “bundles” of fibers having conventional deniers in the proximity of the microporous surface (i.e. the “visible” surface of the final product), which makes the homogeneous appearance of high-quality suede-like artificial leather impossible.
If textile structures could be used with no twisted yarns, the above conveyance phenomena would be limited to single fibers, and the visual and tactile impact would certainly be lower that that due to the presence of said bundles.